That is because they came to believe, and then renounced (their faith). So their hearts were sealed; and now they do not understand.
इस सबब से कि (ज़ाहिर में) ईमान लाए फिर काफ़िर हो गए, तो उनके दिलों पर (गोया) मोहर लगा दी गयी है तो अब ये समझते ही नहीं